1. Field of the Invention
This invention relates to a method for the production of methacrylic acid by the catalytic vapor-phase oxidation of isobutylene and/or tertiary butanol. More particularly, this invention relates to a method for producing methacrylic acid from isobutylene and/or tertiary butanol by the two-step continuous reaction, namely a first step reaction which subjects isobutylene and/or tertiary butanol to catalytic vapor-phase oxidation with molecular oxygen and mainly produces methacrolein and a second step reaction which converts this methacrolein into methacrylic acid. Owing to this invention two-step continuous reaction, the trouble possibly encountered in the second step reaction, i.e. the occlusion of the catalyst bed with an occlusive substance contained in the gas formed by the first step reaction, is precluded and the production of methacrylic acid is obtained smoothly and technically stably.
2. Description of the Prior Art
In the production of methacrylic acid by the catalytic vapor-phase oxidation method from isobutylene and/or tertiary butanol, the two-step oxidation reaction is generally adopted which comprises converting isobutylene and/or tertiary butanol by the catalytic vapor-phase oxidation into methacrolein (hereinafter this reaction will be referred to as the "first step reaction" and the catalyst used therein as the "first step catalyst") and subsequently converting the methacrolein by the catalytic vapor-phase oxidation into methacrylic acid (hereinafter this reaction will be referred to as the "second step reaction" and the catalyst used therein as the "second step catalyst").
The catalyst to be used in the first step reaction is generally a multi-element type oxide catalyst containing molybdenum, bismuth, an iron. When the catalytic vapor-phase oxidation of isobutylene and/or tertiary butanol is carried out in the presence of a catalyst of this type, the reaction in addition to forming methacrolein as a main product producer by product compounds of relatively high boiling points such as maleic acid and terephthalic acid and further entails evolution of a gas containing polymers and tarry substances. When the reaction gas containing such substances is supplied in the unmodified form to the second step reaction, these substances induce clogging of the piping and occlusion of the packed bed of the second step catalyst and consequently cause aggravation of pressure loss, degradation of catalytic activity, and impairment of the selectivity of the reaction for methacrylic acid. These troubles occur frequently when the feed volume or feed rate of isobutylene and/or tertiary butanol is increased or the concentration of isobutylene and/or tertiary butanol is increased for the purpose of ensuring a large output of methacrylic acid.
Among the methods heretofore proposed and adopted for the prevention of these troubles, a method which comprises periodically discontinuing the reaction, removing from the gas inlet side of the second step catalyst an inert substance such as, for example, ceramic balls packed therein for preventing the catalyst bed from occlusion or precluding the loss of catalytic activity, and replacing a new supply of the inert substance, a method which comprise separating methacrolein from the gas produced by the first step reaction and elaborately feeding the separated methacrolein to the second step reaction thereby optimizing the process of oxidation, and a method which comprises diluting the feed gas to a concentration lower than normally required and ensuring a desired decrease in the concentration of by-products in the product of reaction are well known.
These methods, however, are invariably not close to proving satisfactory from the economic point of view because they are complicated and expensive. Further, a method which recycles as an inert gas the waste gas emanated from the reaction gas for the purpose of lowering the oxygen contents of the reaction gases formed in the first step and second step reactions to the fullest possible extent for the purpose of preventing excessive oxidation has found popular acceptance. In this connection, a method which, for the purpose of preventing the catalyst bed from occlusion and precluding the loss of catalytic activity, lowers the concentration of minute solid particles in the waste gas being recycled to the first step reaction to the fullest possible extent (Japanese Patent Laid-Open SHO 56(1981)-113,732), a method which, for the purpose of preventing the piping intervening between the sites of the first step and second step reactions from clogging, retains the portion of the piping at a temperature exceeding the boiling point of maleic anhydride or devises means for extremely increasing the linear speed of gas (Japanese Patent Laid-Open SHO 50(1975)-126,605), and a method which represses the occlusion of the second step catalyst with a solid matter from the first step reaction vessel by giving a specific form to the catalyst used in the second step reaction and consequently increasing the void ratio in the catalyst bed (Japanese Patent Laid-Open SHO 61(1986)-221,149) have been proposed. These methods also are not close to proving fully satisfactory for commercial operation.
Now that the development of the second step catalyst has advanced so much as to promise a reduction in the reaction temperature and an addition to the magnitude of load, the measure for preventing the trouble of clogging of the second step reaction vessel on the gas inlet side has been gaining all the more in significance.
An object of this invention, therefore, is to provide for the production of methacrylic acid an economically advantageous method free of the drawbacks of the prior art described above.
The inventors have found that, in the production of methacrylic acid from isobutylene and/or tertiary butanol by the catalytic vapor-phase oxidation using molecular oxygen and comprising a first-step reaction and a second step reaction, the occlusion of the second step catalyst bed with the by-products entrained by the gas produced in the first step reaction can be efficiently precluded and consequently the reaction performed on a commercial scale can be performed smoothly and stably by keeping a rodlike or platelike insert set in the reaction tube of the second step reactor on the gas inlet side thereof. This invention has been perfected as the result.